The present invention relates in general to presence detectors using tags and to identification systems using labels and more particularly to such systems where tags and labels are recorded and/or decoded holographically in real time. The system of the invention is characterized by its use of ordinary incoherent light to encode and decode labels and thusly resulting in the reduction of the complexity and costs of labeling systems.
Non-holographic non-laser prior art scanning systems have been used to identify objects, for example as shown in U.S. Pat. No. 3,225,177 "Mark Sensing" issued Dec. 21, 1965 to F. Stites and U.S. Pat. No. 3,744,026 "Optical Label Scanning" issued July 3, 1973 to G. Wolff. These patents describe a coded label as a vertical array of substantially parallel, horizontally oriented strips of various widths arranged in accordance with a preestablished code. An incoherent light beam is used to scan strips for providing amplitude and pulse-width modulated waveforms which can be detected and decoded to provide the information of each label. While these systems have performed well they do have a number of disadvantages. They are constrained by Rayleigh optics which limits the amount of coding which can be provided in a label of given size. They are also limited by the scanning rate of the light beam scanning the strips in the label, by the speed of movement of the label in the direction of travel of the object, and by the reflectivity and contrast of labels which, once encoded, cannot be altered. Their signal-to-noise ratio is relatively low and they have no processing gain. Moreover, they must use intense light beams and the decoding of labels is a relatively complex and costly operation. The non-holographic non-laser prior art scanning systems have been used in vehicular and railroad label reading applications.
Non-holographic laser prior art scanning systems have also been used to identify objects, for example as shown in U.S. Pat. 3,889,102 "Off-Axis Circular Coordinate Optical Scanning Device and Code Recognition System" issued June 10, 1975 to J. Dalquist and in the paper by E. Ulmer "Automatic Data Capture of Point-of-Sale Systems" appearing in Session 8 of the IEEE International Convention and Exposition Proceedings, Mar. 26-29, 1974. Although these systems use the laser replacing the non-laser as an illumination source, they do not take advantage of its coherent feature and, as a consequence, suffer the same limitation as do the non-laser systems.
The use of the laser in non-holographic label reading applications has not been without its problems. Lasers are complex devices for many practical applications outside the laboratory. Apart from their being constrained to a limited depth of focus and field of view they must be restricted as radiation hazards. Thus, only low powered lasers may be utilized and these produce weak signals. Their associated photodetectors are subject to photon noise from nearby lights and their use provides no processing gain since the detected signals from individual strips or bars in a label are essentially the detection of unmodulated signals in noise and which, therefore, must compete with the high reflectivity and clutter of the label background. As a result of the low signal-to-background clutter and noise they must compensate by having sophisticated receivers and signal decoders.
It is also possible to encode and decode identification labels holographically, for example as shown in U.S. Pat. No. 3,552,853 "Holographic Identification System" issued Jan. 5, 1971 to H. Saders et al., and in U.S. Pat. No. 3,894,756 "Identification Card Having a Reference Beam Coded Hologram" issued July 15, 1975 to J. Ward. These patents describe the coded label as a hologram. A laser is used to both encode and decode labels. Alternative to the laser, a pinhole, color filter and polarizer can be placed in front of an ordinary incoherent source to obtain coherent energy. In what follows, th combination of a pinhole, color filter and polarizer with an incoherent source to produce coherent light, while inefficient of its use of energy, will be considered to be the equivalent of a laser and both therefore should be distinguished from an incoherent source which illuminates labels directly.
The terms coherent and incoherent are used herein to describe the temporal and spatial characteristics of light. There are two types of coherence, temporal and spatial coherence, and light is said to be coherent when it possesses both types and is said to be incoherent when it lacks both types. Temporally coherent light is light that is monochromatic, i.e., it has only one wavelength. Spatially coherent light is light that is derived from a point source or is capable of being focused to a point. Today the laser is the usual source of coherent light although coherent light can also be obtained from incoherent sources as it was done routinely before the laser was invented. Spatial coherence can be achieved, for example, by placing a pinhole in front of an incoherent source so the light comes from a point. Temporal coherence can be achieved by placing a color filter in front of the incoherent source so that only a narrow band of the light is transmitted. Additionally, polarized light can be achieved by using a polarizer. Each process however involves throwing away by far the greater part of the light and therefore an extremely intense source of incoherent light is needed to generate a very small amount of coherent light. The use of a combination of an incoherent source with pinholes, color filters and polarizers is shown in the systems of H. Sanders and J. Ward. It is a purpose of the invention to replace coherent sources, including lasers and equivalent combinations of incoherent sources with pinholes, color filters and polarizers, by incoherent ones in holographic labeling systems.
While suggestive of the use of a new and potentially powerful technique of using holography in labeling systems, the prior art holographic labeling systems with lasers suffer the same restrictions as do the non-holographic laser labeling systems. And, the effort by the prior art to overcome the problems associated with lasers through the replacement of the laser by the combination of an incoherent light source with a pinhole, color filter and polarizer is an obvious waste of illumination energy.
From the above discussion it is clear that there are two basic approaches to labeling systems used for the identification of objects; non-holographic and holographic encoding and decoding systems. While both coherent and incoherent sources have been utilized by the prior labeling art in non-holographic encoding and decoding systems (mechanical scanning of conventional labels), the prior labeling art restricts holographic encoding and decoding of labels exclusively to coherent sources including lasers and equivalent combinations of incoherent sources with pinholes, color filters and polarizers It is a purpose of the present invention to provide holographic labeling systems using other than coherent sources.
In the present holographic art there are three types of holograms which can be recorded and reconstructed with incoherent light rather than requiring monochromatic or even coherent light. These are volume holograms and thin amplitude and phase holograms which can be recorded and reconstructed in a manner in which substantially all of the available energy is utilized and contributes to the image resolution and brightness of the holographic reconstructed image. Unlike the hologram art, the labeling art lacks the capability to encode and decode labels with incoherent light so that most of the available energy is utilized. The present labeling art is confined to the use of coherent light or its equivalent combination of incoherent light with a pinhole and color filter in which latter use a substantial portion of the incoherent light available is not used.
The present invention is directed to apparatus and method which makes it possible to encode and decode volume and surface type labels, which may be redundant or non-redundant holograms, with incoherent light in a manner in which substantially all of the available light is used and contributes to the resolution and brightness of the decoded images of labels. Briefly, this is done by applying the well known teachings of the holographic art for using incoherent light to the labeling art.
There are many advantages in being able to provide labeling systems using holography. First, the high information capacity of holographic labels can be used to substantially increase the signal-to-noise ratio of detected signals and for encoding multiple depths of focus, aspects and orientations of labels to increase the range and field of view. Second, labels are unobservable and illegible and can be further coded to provide additional security of access. Third, transitory as well as permanent labels may be provided. Fourth, encoders and decoders spatially expand their energy and cannot be limited as radiation hazards as do spot beam lasers and therefore need not be restricted to low power operation with consequent low signal-to-noise ratios for decoding signals and the need for sophisticated receivers and signal decoders. Fifth, associated photo detectors are least subject to photon noise from nearby lights and thereby further relaxing signal decoding requirements. Sixth, magnification can be obtained by several means, for example by using a lower wavelength to decode labels than the wavelength used for encoding. Seventh, amplification can also be obtained by several means, for example by using a relatively high intensity reference and/or read beam of wave energy, while using a relatively low intensity signal beam so that the image of the label appears brighter than the actual illumination with the relatively low intensity wave energy. Finally, by utilizing different wave lengths to encode and decode labels, either up-conversion or down-conversion can be obtained, thereby permitting encoding labels at acoustic, microwave, and infra-red frequencies and decoding labels in visible light. These and others are some of the advantages of implementing labeling systems using holography.
There are also many advantages in being able to provide holographic labeling systems without coherent light sources. First, the use of incoherent light increases the image brightness compared to the use of a coherent source. Second, the use of incoherent light in the invention does not throw away energy compared to use of same in the prior art, thereby permitting the full use of all energy from the incoherent light source. Third, the relaxed coherency requirement using incoherent source illumination means the label can be moved while it is being decoded thereby expanding the flexibility of operating labeling systems. Finally, fourth, incoherent sources are less complex and have lower costs compared to lasers, thereby permitting more practical and universal applications. These and others are some of the advantages of implementing labeling systems with incoherent light sources.
In the foregoing the term holographic labeling system has been used to denote the holographic encoding and decoding of labels Clearly, while labels can be encoded with information for identifying objects individually they can be equally encoded for determining the presence of objects, for example by encoding the same information on all labels for producing a holographic tagging system, i.e., a system which detects the presence of objects which have tags. It should be understood therefore that tagging systems are but variants of identification systems.
Accordingly, it is an object of this invention to provide improved holographic tagging and labeling systems.
It is a more specific object of the invention to provide holographic tagging and labeling systems which drastically reduce the coherence requirement for encoding and decoding labels.
It is another object of the invention to provide holographic tagging and labeling systems which utilize substantially all the energy of sources used to encode and decode labels.
It is another object of the invention to provide holographic tagging and labeling systems which drastically increase the signal-to-noise ratio o f signals.
It is another object of the invention to provide holographic tagging and labeling systems which drastically reduce the complexity and cost of encoding and decoding labels.
It is another object of the invention to provide in conjunction with one or more of the foregoing objects tagging and labeling system having a relaxed field of view requirement for encoding and decoding labels, a relaxed orientation requirement for encoding and decoding labels, unobservable and illegible labels, permanent and transitory labels, reduced radiation hazard requirements for encoding and decoding labels, reduced effects of photon noise in receivers, relaxed motion requirements for encoding and decoding labels, and the ready magnification, amplification and frequency shifting of signals.